In recent years, there has been widely used an image display device such as a projection display for displaying an image on a screen. Generally, a lamp light source is used in the projection display. The lamp light source, however, has drawbacks that the life is short, the color reproducing area is limited, the light source area is large, and the light use efficiency is low.
In order to solve the above drawbacks, there is proposed an idea of using a laser light source as a light source in an image display device. Since the laser light source has a long life and a strong light directivity, as compared with a lamp light source, the light use efficiency can be easily increased. Since the laser light source emits monochromatic light, the color reproducing area is large, and a clear image can be displayed. Further, as compared with a conventional lamp light source, the laser light source can be miniaturized, and an optical component can be miniaturized because the laser light source is a point light source. Thus, it is conceived that the laser light source can be applied to a portable projector, which has not been available in the conventional art.
However, in a projection display incorporated with a laser light source, speckle noise may be generated resulting from high interference of laser light. The speckle noise is roughly classified into speckle noise in a diffraction field, wherein a pattern generated in an optical system is displayed on a display plane; and speckle noise in an image field, wherein laser light is scattered on a display plane, and the scattered light turns into micro grain noises in the eyes of a viewer. The former speckle noise in a diffraction field causes a problem in an exposing machine and an illuminating optical system, and both of the speckle noise in a diffraction field and the speckle noise in an image field cause a problem in a projection display incorporated with a laser light source.
It is necessary to provide an optical system for uniformly illuminating a display apparatus as required in order to miniaturize a projection display, while utilizing a characteristic of a point light source, which is different from an optical system incorporated with a conventional lamp or a conventional LED as a light source. It is necessary to increase the light use efficiency in order to miniaturize a display apparatus, in view of a problem relating to an exothermic operation.
Heretofore, there have been proposed various improvements in a display apparatus incorporated with a laser light source to reduce speckle noise or uniformly illuminate the display apparatus.
For instance, in patent literature 1, uniform illumination of a spatial modulator and speckle noise reduction are performed by combining a beam shaping optics constituted of a laser light source, a beam expander, two sets of fly-eye lenses, a condenser lens, and a field lens; and a movable diffuser. However, the optical system proposed in patent literature 1 has a size as large as the size of an optical system required in a lamp light source, and accordingly, miniaturization of the optical system is difficult.
Patent literature 2 proposes an arrangement constituted of a combination of lens arrays having a shortened optical path, and a condenser lens in order to shorten the distance between fly-eye lenses in an exposing machine. However, in patent literature 2, sufficient miniaturization of an optical system has not been achieved, and the number of parts is increased.
There is also proposed an idea of utilizing an element applied with processing in the order of the wavelength such as a diffraction element or an HOE element, as beam shaping means for a laser light source. However, the aforementioned elements have a restriction on the use wavelength and the incident angle, and it is difficult to make the light intensity uniform or perform accurate beam shaping with respect to a multi-wavelength light source or plural light sources.
The conventional proposals have failed to miniaturize a laser illuminating device incorporated with a laser light source, and an image display device incorporated with the laser illuminating device, while removing speckle noise and performing uniform illumination. Thus, there is a serious problem to overcome in fabricating a portable projector utilizing a characteristic of a laser light source. Further, it is necessary to realize speckle noise removal, uniform illumination, and miniaturization of the device with use of a laser light source for emitting laser light of three or more colors in order to display a color image.
Patent literature 1: JP 2003-98476A
Patent literature 2: JP 2006-309207A